Generally speaking, a system for processing a signal output from a signal generator includes a TV and display monitor. These devices have a filter circuit so that the signal output from a signal generator is supplied stably.
From now on, there will be explained a conventional display monitor having a noise filter in order to eliminate a noise component contained in the sync signal output from a personal computer (PC) signal generator.
FIG. 1 is a block diagram of an internal circuit of a generally used monitor. As in FIG. 1, PC 100 processes a key signal produced from a keyboard (not shown) through CPU 110, generating data. This data is received in video card 120, and is processed into video signals R,G,B. Video card 120 outputs horizontal and vertical sync signals H-SYNC and V-SYNC for synchronizing output video signals R,G,B.
These output video signals R,G,B from video card 120 of PC 100 are applied and amplified in video AMP 280 of display monitor 200, and are then sent to a cathode ray tube (CRT) 290. Horizontal/vertical sync signal H/V-SYNC output from video card 120 of PC 100 is received by noise filter 210 of a video input (not shown). Noise filter 210 receiving horizontal/vertical sync signal H/V-SYNC filters its noise.
The horizontal/vertical sync signal H/V-SYNC noise-filtered through noise filter 210 is sent to horizontal/vertical signal processor 220a incorporated in microcomputer 220. Microcomputer 220 determines the resolution of output video signals R,G,B from video card 120 of PC 100 according to horizontal/vertical sync signal H/V-SYNC applied to horizontal/vertical signal processor 220a.
Microcomputer 220 outputs an oscillation signal through horizontal/vertical oscillation signal processor 220a according to the determination result. The oscillation signal output from horizontal/vertical oscillation signal processor 220a is received in horizontal/vertical oscillation signal output portion 230. The horizontal/vertical oscillation signal portion 230, receiving the oscillation signal, outputs horizontal and vertical oscillation pulses according to the oscillation signal applied.
The horizontal oscillation pulse output from horizontal/vertical oscillation signal output portion 230 is received in horizontal drive circuit 240 so that it supplies a drive current sufficient to enable horizontal output circuit 250 to switch. The horizontal output circuit 250, receiving the drive current output through horizontal drive circuit 240, switches according to the drive current applied so that a horizontal sawtooth wave current is produced in horizontal deflection yoke H-DY.
A vertical drive circuit 260 receives the vertical oscillation pulse output from horizontal/vertical oscillation signal output portion 230, and supplies a drive current to vertical output circuit 270. The vertical output circuit 270, receiving the drive current output from vertical drive circuit 260, provides a vertical sawtooth wave current to vertical deflection yoke V-DY according to the drive current applied.
The CRT 290, receives the video signals R,G,B output from video AMP 280 according to the cycle of sawtooth wave current produced in horizontal and vertical deflection yokes H-DY and V-DY, and displays an image with video signals R,G,B according to the cycle of the horizontal and vertical sawtooth waves.
Noise filter 210 for removing noise contained in horizontal/vertical sync signal H/V-SYNC output from video card 120 of PC 100 will be described below in more detail.
When noise produced through PC 100 and signal cable (not shown) is mixed in the horizontal and vertical sync signals output from video card 120 of PC 100, the deflection becomes unstable so that an image is displayed unstably on CRT 290. Especially, if noise is mixed in horizontal sync signal H-SYNC, the image is dispersed or becomes unstable due to the noise.
In order to remove such distortion and noise mixed in horizontal sync signal H-SYNC, noise filter 210 is provided with a resistor and a capacitor between the horizontal sync signal input stage (not shown) and ground. Noise filter 210 using the resistor and capacitor cannot be employed in a product of horizontal sync signal frequency below 48 KHz, and thus it is disadvantageous in broad usage.
In order to overcome this problem, according to the conventional art, noise filter 210 is formed with a buffer IC (not shown) to eliminate the noise component contained in horizontal sync signal H-SYNC. Noise filter 210 using the buffer IC unfavorably requires a driving power (generally +5V) output from the secondary side of the power circuit (not shown) under conditions where the display monitor 200 normally operates.
Further, filtering is performed only within a range (generally 1.5V-3.15V) limited to noise immunity prescribed in the buffer IC specification. Above that value, filtering is not carried out so that noise over 3.15V is not removed.